The present invention relates to a semiconductor device and a method for fabricating the device and more particularly relates to a semiconductor device including low resistance heavily doped regions and a method for fabricating the device.
Recently, the number of devices integrated for a semiconductor integrated circuit has been increasing steeply and MIS semiconductor devices have been downsized drastically. Thus, to suppress a short channel effect and increase the drivability thereof, a gate electrode has to have its resistance decreased and an extended heavily doped layer, which is defined under the edges of the gate electrode as parts of source/drain regions, needs to have its resistance decreased and its junction depth reduced. Hereinafter, a known method for fabricating a p-channel MIS semiconductor device including an extended heavily doped layer will be described with reference to FIGS. 6A through 6D. FIGS. 6A through 6D are cross-sectional views illustrating known process steps for fabricating an MIS semiconductor device including an extended heavily doped layer.
First, in the process step shown in FIG. 6A, a trench isolation film 52 is formed in parts of an n-type semiconductor substrate 51 to define an active region. Then, a gate electrode 54 of polysilicon is formed over the active region in the semiconductor substrate 51 with a gate insulating film 53 interposed between them. Thereafter, ions of boron as a p-type dopant 55 are implanted using the gate electrode 54 and the isolation film 52 as a mask to form an ion-implanted layer 56 as a prototype for an extended heavily doped layer.
Next, in the process step shown in FIG. 6B, the substrate is heated to an elevated temperature for a short time. Specifically, the substrate is subjected to a first rapid thermal annealing (RTA) process under the conditions including 900xc2x0 C. and 10 sec., for example, to activate the dopant existing in the ion-implanted layer 56. In this manner, an extended heavily doped layer 56a is formed.
Subsequently, in the process step shown in FIG. 6C, a silicon nitride film is deposited over the substrate under the conditions including 700xc2x0 C. and 20 min., for example. Then, the silicon nitride film is etched anisotropically to form a sidewall 57 of silicon nitride on the side faces of the gate electrode 54. Thereafter, ions of boron as a p-type dopant 58 are implanted into the substrate using the gate electrode 54, sidewall 57 and isolation film 52 as a mask to form an ion-implanted layer 59 as a prototype for a heavily doped source/drain layer.
Then, in the process step shown in FIG. 6D, the substrate is subjected to a second RTA process under the conditions including 1100xc2x0 C. and 0.5 sec., for example, to activate the dopant existing in the ion-implanted layer 59. In this manner, the extended heavily doped layer 56a and a heavily doped source/drain layer 59a are defined.
In a known method, an MIS semiconductor device is fabricated in the following manner to improve its drivability; the boron ions 55 are implanted at a low accelerating voltage in forming the ion-implanted layer 56 so that the extended heavily doped layer 56a has a shallow junction depth. Further, in forming the ion-implanted layer 56 by this process, the implant dose of the boron ions 55 is set as high as possible to decrease a parasitic capacitance produced between the extended heavily doped layer 56a and heavily doped source/drain layer 59a. 
However, the MIS semiconductor device obtained by the known method has the following drawbacks.
Firstly, we found that if a dopant was implanted as much as possible to reduce the resistance of the extended heavily doped layer, the resistance rather increased in the final structure.
Secondly, we also found that after the structure had gone through many process steps succeeding the second RTA process, the junction depth of the extended heavily doped layer had increased very much in the resultant MIS semiconductor device. The greater the junction depth, the more likely punch-through or short channel effect occurs.
It is therefore an object of the present invention to clarify why those unfavorable phenomena occur and thereby provide a semiconductor device including low resistance heavily doped regions and a method for fabricating the device.
An inventive semiconductor device includes at least one heavily doped region that has been formed by introducing a dopant into a semiconductor layer. A maximum concentration of the dopant existing in the heavily doped region is at least greater than a predetermined percentage of a solid solubility of a first annealing process for activating the dopant and equal to or less than a solid solubility of a second annealing process that needs to be performed at the highest temperature after the dopant has been introduced into the semiconductor layer by an ion implantation process.
According to the present invention, almost all of a dopant existing in the heavily doped region can be activated. Thus, it is possible to suppress the heavily doped region from having its resistance increased by the deactivation (e.g., clustering or precipitation) of the dopant. As a result, a low resistance semiconductor device can be obtained. Further, the dopant may be introduced for an inventive semiconductor device at a dose lower than the dose for a known semiconductor device. Therefore, the junction depth of the dopant can be reduced and a semiconductor device with high drivability can be obtained.
In one embodiment of the present invention, almost all of the dopant existing in the heavily doped region has preferably been activated.
In another embodiment, the maximum concentration of the dopant may be 90 to 100% of the solid solubility at a process temperature of the first annealing process. In such an embodiment, almost all of the dopant can be activated through the first annealing process. Thus, no inactive dopant atoms will newly deactivate the activated dopant atoms even if annealing processes are carried out at low temperatures for a long time after that. Accordingly, the dopant can be activated with more certainty.
In an alternative embodiment, the maximum concentration of the dopant may be greater than the solid solubility at a process temperature of the first annealing process and may be 90 to 100% of the solid solubility at a process temperature of the second annealing process. Then, the heavily doped region may contain the dopant at the highest possible concentration that the second annealing process can cope with.
In still another embodiment, the inventive semiconductor device may include: a gate electrode, which has been formed over the semiconductor layer with a gate insulating film interposed between the gate electrode and the semiconductor layer; an extended heavily doped layer, which has been defined as the heavily doped region in parts of the semiconductor layer that are located beside the gate electrode; a sidewall, which has been formed on the side faces of the gate electrode; and a heavily doped source/drain layer, which has been defined in parts of the semiconductor layer that are located beside the sidewall to surround the outer periphery of, and be in contact with, the extended heavily doped layer, and which has a junction deeper than the junction of the extended heavily doped layer.
In yet another embodiment, the semiconductor layer may be a gate electrode of polysilicon and the gate electrode may include the heavily doped region. Then, an MIS semiconductor device can have its resistance reduced.
An inventive method for fabricating a semiconductor device includes the step of a) introducing a first dopant into at least part of a semiconductor layer by an ion implantation process, thereby forming a first ion-implanted layer in the part. The method further includes the step of b) subjecting the first ion-implanted layer to a first annealing process. And the method further includes the step of c) subjecting the semiconductor layer to a second annealing process, thereby forming a heavily doped region out of the first ion-implanted layer after the step b) has been performed. In the step a), the first dopant is introduced at such a dose that a maximum concentration of the first dopant when the first annealing process is over will be greater than a predetermined percentage of a solid solubility at a process temperature of the first annealing process and that a maximum concentration of the first dopant when the second annealing process is over will be equal to or less than a solid solubility at a process temperature of the second annealing process. In the step c), the second annealing process is performed at the highest process temperature in all process steps succeeding the step a).
According to the present invention, almost all of a dopant existing in a heavily doped region can be activated through annealing processes. Further, the dopant may be introduced in the inventive method at a dose lower than that adopted by a known method. Therefore, the junction depth of the dopant can be reduced and a semiconductor device with high drivability can be obtained.
In one embodiment of the present invention, almost all of the first dopant existing in the first ion-implanted layer is preferably activated through the first annealing process.
In another embodiment, the maximum concentration of the first dopant in the step b) may be 90 to 100% of the solid solubility at the process temperature of the first annealing process. In such an embodiment, almost all of the dopant can be activated through the first annealing process. Thus, no inactive dopant atoms will newly deactivate the activated dopant atoms even if annealing processes are performed at low temperatures for a long time after that. Accordingly, the dopant can be activated with more certainty.
Alternatively, the maximum concentration of the first dopant in the step b) may be greater than the solid solubility at the process temperature of the first annealing process. In the step c), the maximum concentration of the first dopant may be 90 to 100% of the solid solubility at the process temperature of the second annealing process. Then, the dopant may be introduced for the heavily doped region at the highest possible implant dose that the second annealing process can cope with.
In still another embodiment, before the step a) is performed, a gate electrode may be formed over the semiconductor layer with a gate insulating film interposed between the gate electrode and the semiconductor layer. In the step a), the first ion-implanted layer may be formed in parts of the semiconductor layer that are located beside the gate electrode by performing the ion implantation process using the gate electrode as a mask. The method may further include the steps of: forming a sidewall on the side faces of the gate electrode after the step b) has been performed; and introducing a second dopant by an ion implantation process using the gate electrode and the sidewall as a mask, thereby forming a second ion-implanted layer. The step c) may further include forming an extended heavily doped layer and a heavily doped source/drain layer out of the first and second ion-implanted layers, respectively, so that the extended heavily doped layer is located below edges of the gate electrode and that the heavily doped source/drain layer surrounds the outer periphery of, and is in contact with, the extended heavily doped layer and has a junction deeper than the junction of the extended heavily doped layer. The extended heavily doped layer may be the heavily doped region.
In yet another embodiment, the semiconductor layer may be a gate electrode of polysilicon and the heavily doped region may be defined in the gate electrode. Then, an MIS semiconductor device can have its resistance reduced.
In yet another embodiment, the first and second annealing processes may be performed as a single process step. Then, a low resistance semiconductor device with high drivability can be obtained through simpler process steps.
In yet another embodiment, after the first dopant has been introduced into the semiconductor layer in the step a), almost all of the first dopant is preferably activated by performing the first annealing process in the step b). The second dopant is preferably introduced into the semiconductor layer by the ion implantation process before the step c) is performed. And in the step c), the second dopant is preferably activated through the second annealing process, thereby forming the extended heavily doped layer and the heavily doped source/drain layer. Then, the first and second dopants can be activated with more certainty.